1. Field
Disclosed herein are methods and apparatus for removing elemental mercury and/or other liquid contaminants from a geological subsurface substrate or formation by mobilizing and passively collecting the mercury and/or other liquid contaminants through, e.g., a soil column, and collecting it in pipes and sumps.
2. Description of Related Art
Mercury is a silver-white, heavy liquid at room temperature that forms small, compact droplets when released into the environment. The mercury in these droplets contains a high vapor pressure, causing it to evaporate and become easily inhalable. Contamination of soil and subsurface geological formations by elemental mercury may result from past use of a particular site for a variety of industrial uses, such as mining and smelting, burning of fossil fuels, industrial production of caustic, waste incineration, pharmaceutical production, etc. Mercury is often encountered in such environments in elemental form.
Because mercury, and in particular, elemental mercury, is considered to be a hazardous material that is easily absorbed by inhalation, as well as leachable into groundwater, and because mercury can potentially cause harmful effects to the central nervous system, kidney toxicity, etc., its presence in the soil of sites to be used by humans is generally undesirable, and has been subject to environmental regulatory control. One hundred seventy three Superfund Sites list mercury as a contaminant of concern in the soil of the site. Nonlisted wastes are defined as hazardous wastes if the concentration of mercury in the waste extract exceeds 0.2 mg/L using the Toxicity Characteristic Leaching Procedure (TCLP) under the toxicity criteria of the Resource Conservation and Recovery Act (RCRA). The U.S. Environmental Protection Agency has set a Land Disposal Restriction (LDR) standard for mercury contamination in soil that requires treatment to reduce by 90% of the original concentration of mercury in the soil, or to a level of 2.0 mg/L using the TCLP procedure. See Treatment Technologies for Mercury in Soil, Waste, and Water, U.S. Environmental Protection Agency, Office of Superfund Remediation and Technology Innovation, August, 2007, incorporated herein by reference.
Accordingly, several techniques have been developed in attempts to remediate soil or other particulate material that has been contaminated by mercury or other contaminants. These include solidification/stabilization, where the contaminant is physically bound or enclosed within a stabilized mass, and thereby converted into a less soluble, less mobile form. Another technique is soil washing/acid extraction, which involves suspending soil in a wash solution and separating the fine particulates therefrom. Contaminants that preferentially sorb onto the fines can be separated from the suspension, thereby reducing the concentration of the contaminant in the remaining soil. This procedure may involve the contacting of the soil with an extracting chemical, such as hydrochloric or sulfuric acid. Another technique is thermal desorption/retorting, which involves the application of heat and/or reduced pressure to volatilize mercury or other contaminant from the soil, and collection of the vaporized contaminant by condensation. Additional scrubbing processes may be needed to treat off-gas resulting from this technique. Finally, vitrification is a high temperature treatment to reduce the mobility of contaminants by incorporation into a chemically durable, leach-resistant vitreous mass.
Each of these techniques has the drawback that it is relatively complex and expensive. Since liquid contaminants like elemental mercury can penetrate to a significant depth below the surface in the soil of a contaminated site, these techniques require the removal and processing of significant volumes of soil. Some of the techniques, such as soil washing/acid extraction, is primarily used to treat soils with low clay content, and which can be separated into a highly contaminated fines fraction and a less contaminated sands fraction. Thermal desorption/retorting is also not suitable for soils with high clay content, and requires additional processing to deal with the off-gases produced. Both of these techniques require removal of the soil from the site of the contamination. Solidification/stabilization and vitrification both change the nature and character of the soil that has been contaminated, rather than simply decontaminating it, and result in a significant volume of material that must be disposed of in a landfill. All of these procedures are capital intensive. Accordingly, there remains a need in the art for an inexpensive, simple method for removing mercury and/or other liquid contaminants from soil or other solid particulate materials.
In addition, the identification of the site of contamination, and thus the location where remediation efforts are most likely to be valuable, can be difficult, sometimes requiring analysis of samples of soil and other materials for the presence of mercury, which can be time consuming and expensive. Accordingly, there remains a need in the art for an inexpensive, fast, reliable method for determining the presence of mercury in soil and subsurface structures.